NHTSA "has effectively given up on rule making unless specifically required by statute, focusing instead on its statutory power to force the recall of motor vehicles that contain 'defects' related to safety performance," says Thomas McGarity, a University of Texas law school professor. This creates policy gradually, but doesn't do it proactively.
A big reason NHTSA has given up on proactive rule making is the tenacity with which automakers fight safety regulations. In recent years, regulations ordered by congress have been significantly delayed, according to The Detroit News. Take the rear-view camera legislation that was passed by Congress in 2008: those cameras still don't come on every car, but NHTSA says 100 children under age 5 continue to be killed per year in backup crashes.
Senator Richard Blumenthal (D-Conn.) reportedly says that NHTSA "had to do by recall what it should have been able to do by rule," because of safety regulation delays. He says rules "were suggested to NHTSA by automobile safety advocates but went nowhere, only to come up again when defective automobiles have had to be removed from the road."